There are known heat insulation mixtures which contain finely divided to highly dispersed materials as the main component and which, because of their extraordinarily small heat conductivity, are used to a considerable extent as excellent heat insulation agents.
These types of heat insulation mixtures can be employed in loose ballast form with suitable jacketing according to German OS No. 2904412, pages 4-5, or as compacted masses. However, generally the heat insulation mixtures are used as granulates compacted, for example, according to Belgian Pat. No. 857,490 or they are used in plates or molded pieces (compacts) of fixed dimension according to German AS No. 2036124. In the case of compacting, there are added to this heat insulation agent in small amounts a binder or preferably inorganic fibers to increase the mechanical strength.
Heat insulation mixtures which have an extraordinarily good insulating effect at high temperatures are impervious to the heat irradiation which occurs, whose wavelength is predominantly in the infrared range. For this purpose, there are incorporated into the heat insulation mixture opacifiers, which have the properties of reflecting, absorbing, and scattering the heat radiation which occurs, whereby in all cases an interplay of all properties is present.
The opacifier used most frequently until now is titanium dioxide in the modifications rutile, anatase, or brookite. According to Bruno U.S. Pat. No. 2,808,338, rutile scatters the heat radiation. Likewise, according to German AS No. 2524096, there can be used as opacifier ilmenite, as well as manganese oxide, iron oxide or chromium oxide; and according to German OS No. 2754956, there can be used silicon carbide, boron carbide, tantalum carbide, and tungsten carbide as well as graphite and carbon black.
Of the enumerated opacifiers, carbon black and graphite can only be employed at elevated temperatures if the permanent absence of oxygen is guaranteed. Otherwise, the opacifying agent properties of graphite and carbon black are lost through decomposition. Similar is also true for some of the oxides mentioned, especially manganese oxide.
The mentioned known opacifiers, especially the metal oxides, have become very expensive because of the increased price of the raw material. The carbides of silicon, boron, tantalum, or boron are even more expensive than, for example, the previously used opacifiers, such as e.g., titanium dioxide, because of the high production and grinding costs involved.
Consequently, there is a need to find an opacifier which has the same stopping properties as the known opacifiers but can withstand high temperatures and whose cost of use is low.